Method of obtaining enhanced localized thermal interface regions by particle stacking

ABSTRACT

Integrated circuit-chip hot spot temperatures are reduced by providing localized regions of higher thermal conductivity in the conductive material interface at pre-designed locations by controlling how particles in the thermal paste stack- or pile-up during the pressing or squeezing of excess material from the interface. Nested channels are used to efficiently decrease the thermal resistance in the interface, by both allowing for the thermally conductive material with a higher particle volumetric fill to be used and by creating localized regions of densely packed particles between two surfaces.

TECHNICAL FIELD

The present disclosure is concerned with reducing the thermal resistancebetween a heat source and a cooling device and particularly between anintegrated circuit chip and a heat sink or heat spreader. According tothe present disclosure, chip hot-spot temperatures are reduced bycreating regions of higher thermal conductivity in the interface betweenthe chip and heat sink or heat spreader during bondline formation.

BACKGROUND

Thermal interfaces in microelectronics packages are commonly creditedwith a majority of the resistance for heat to escape from the chip tothe attached cooling device (e.g. heat sinks, spreaders and the like).Thus, in order to minimize the thermal resistance between the heatsource and cooling device, a thermally conductive paste, thermal greaseor adhesive that contains thermally conductive particles is commonlyused. On the chip side—(heat source) there usually exists hotspots,areas of higher power density, where most of the processing takes place,which results in a temperature gradient across the chip. These areas ofhigher heat and power density must be kept within a set temperaturerange in order for the chip to perform properly and to pass quality andspecification tests at the end of manufacturing.

Control of temperature distribution has recently been addressed bychanging chip design/architecture. However this requires expensiveredesign of the microprocessor that may influence other operatingparameters and does not address the present issues facing current highperformance microprocessors. Current means to increase the heatdissipation of the microprocessor include mechanical cap standoffs indirect contact between the chip and heat sink (June et al., “UsingCap-Integral Standoffs to Reduce Chip Hot-Spot Temperatures inElectronic Packages”, 2002 Inter Society Conference on ThermalPhenomena); patterned surfaces with protrusions to reduce the thermalpath; or direct water cooling targeted to cool the hot spots usingliquid cooling (http://cooligy.com;http://www.apple.com/powermac/design.html and Bash et al. “Improvingheat transfer from a flip-chip package-Company Business and Marketing”,Hewlett-Packard Journal, August 1997). All of these approaches requireeither complex modifications of the package lids, caps or heat sink, andor expensive water cooling with pumps, fluidic interconnects and heatexchangers.

The aforementioned methods of reducing the thermal resistance betweenthe chip and heat sink all have one thing in common: a thermalpaste/adhesive or other medium softer than the two other surfaces isalways placed between the heat source and sink in order to reduce theeffect of manufacturing and assembly tolerances and to simplify assemblyand reworkability. The material used in these layers is usuallyengineered to have a low as possible thermal resistance.

Accordingly, it would be desirable to provide for reduced thermalresistance between a heat source and cooling device that is bothefficacious and yet not require changes to the microprocessorfabrication process.

SUMMARY OF THE INVENTION

According to the present disclosure, chip hot-spot temperatures arereduced by creating regions of higher thermal conductivity in thethermally conductive interface material during bondline formation.

More particularly, one aspect of the present disclosure relates to amicroelectronic structure comprising

an integrated circuit chip;

a heat sink adjacent the integrated circuit chip; wherein the heat sinkcontains nested channel arrays located opposite the vicinity of hotspots on the integrated circuit chip; and

a thermally conductive composition containing thermally conductiveparticles located between the heat sink and integrated circuit chip andcontaining higher concentration of conductive particles in the nestedchannel arrays.

A further aspect of the present disclosure relates to a method forfabricating a microelectronic structure comprising

providing an integrated circuit chip;

providing a heat sink or cooling device, such as a thermoelectric cooleror microchannel cooler, adjacent the integrated circuit chip; whereinthe heat sink contains nested channel arrays located opposite thevicinity of hot spots on the integrated circuit chip;

providing a fluid thermally conductive composition containing thermallyconductive particles between the integrated circuit chip and heat sink;and

pressing the integrated chip and heat sink together causing thethermally conductive material to flow outward from the vicinity of thecenter and to thereby provide higher concentration of the thermallyconductive particles in the nested channel arrays.

In preferred embodiments, the process provides regions of optimum orsubstantially optimum particle packing and thermal conductivity at thecenter of the channel array.

Still other objects and advantages of the present disclosure will becomereadily apparent by those skilled in the art from the following detaileddescription, wherein it is shown and described only the preferredembodiments, simply by way of illustration of the best mode. As will berealized, the disclosure is capable of other and different embodiments,and its several details are capable of modifications in various obviousrespects, without departing from the disclosure. Accordingly, thedescription is to be regarded as illustrative in nature and not asrestrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates hot spot locations on a sample chip.

FIG. 2 a is a top view of corner to corner velocity gradient andX-shaped particle stacking line as seen through a flat glass substrate.

FIG. 2 b is a cross sectional view of stacking phenomenon along velocitygradient during bondline formation.

FIGS. 3 a-3 c illustrates control of stacking lines (with higherparticle density) obtained by nested channels which are added to thesubstrate.

FIG. 4 shows measured decrease in bondline resistance using nestedchannel cells.

FIG. 5 shows controlling the location of stacking clusters by relativehydraulic diameter and direction of channels.

BEST AND VARIOUS MODES

In order to facilitate a further understanding of the presentdisclosure, reference will be made to the figures, which are presentedfor purposes of illustration and not to limit the disclosure.

According to the present disclosure, the thermal conductivity at desiredlocations can be increased by controlling how particles in the thermallyconductive fluid composition or composite (e.g.—paste, grease oradhesive) stacks—or piles-up during the squeezing of excess materialfrom the interface. Localized particle stacking is controlled using aproperly designed nested micro-channel array on one of the interfacesurfaces such as the heat sink or spreader or an attached coolingdevice. The array of nested channels is designed around the knownhotspot regions of the corresponding chip.

The advantage of this solution is that it further reduces chiptemperatures through only a small modification to the chip cap surfaceand does not require changes to the manufacturing line or the additionof more components to the system such as liquid coolants andmicrochannel heat exchangers.

As previously mentioned, hot spots (areas with higher power density)typically occur where most of the processing takes place. As an exampleof this, FIG. 1 shows the thermal map of an IBM Power5 Microprocessor.The areas of higher power density operate at higher temperatures thanthe rest of the chip. These areas typically operate at 2-4 degreeshigher than the surrounding areas.

Thermal interfaces are typically formed by pressing the heat sink orchip cap onto the backside of the processor chip with a particle filledviscous medium between, which is forced to flow into cavities ornon-uniformities between the surfaces. As the interface material flowsoutward from the center, the edges of the chips are eventually reachedand a velocity gradient develops along a line connecting oppositecorners as shown in FIG. 2 a. The velocity gradient is created due tothe higher pressure drop along the chip center-to-corner path comparedto the center-to-side path. For a relatively uniform gap and pasteviscosity, the pressure drop is primarily a function of the length theviscous material must flow to reach the perimeter of the chip and thusit is highest for the center-to corner path.

During ideal bondline formation, a small volume of interface materialinitially along the velocity gradient would be evenly sheared indifferent directions as the bondline is continually pressed smaller. Fora purely viscous fluid this shearing effect has no impact on thebondline formation. However, for a particle filled interface material,the shearing effect is the direct cause of particle stacking. Whenparticles initially lie on the center-to-corner velocity gradient, theyare pulled in opposite directions and as a result do not move with theviscous matrix but instead begin to pile up as the viscous matrix flowsoutward—forming a mechanical stop preventing further decreases inbondline thickness (see FIG. 2 b).

Along stacking lines observed, there typically exists an excess of highconductivity and well packed particles. Because particle stacking iscaused by the velocity gradient present in rectangular chip squeezeflow, it can be controlled by breaking up the gradients into more finelydistributed regions using micro channels patterned on one of surfaces.

One major difference between the stacking patterns of FIGS. 3 a-3 cabove is the far right substrate does not have a stacking line thatperfectly bisects the interior angles between channels into two equalhalves. The stacking angle can be understood by comparing the pressuredrop of two adjacent channels—if both channels have identical pressuredrops then the interior angle will be equally bisected into two halves.When a difference in pressure drops exists then the stacking angleshifts closer to the channel with higher pressure drop. The pressuredrop of two different channel designs can be predicted using existingfluidic models. This facilitates the use of the channel depth or widthto control the location of the stacking clusters. Typically, thechannels have a depth from, but not limited to, about 10 to about 300micrometer and a width from about 10 to about 300 micrometer being moretypical. The channels are typically located in the heat sink oppositehot spots on the backside of the chip or within the vicinity of thechip. The channels can span the entire chip area, if desired. However,the focal point of the channels or center point where the channels metwould typically be over the hotspot. The particular configuration,location and dimensions of the channels can be tailored by those skilledin the art once they are aware of the present disclosure without undueexperimentation depending upon the desired effect.

The measured decrease in bondline resistance using nested channel cellsis clearly shown in FIG. 4. The term “nested” is use herein to mean ahierarchical system. In this case, it means that there are small sizedchannel networks “nested” within larger channel networks. Note how theresistance decreases, even with higher particle density, using nestedchannels when compared to the sharp increase in resistance using flatsubstrates. Furthermore, rather than attempting to minimize the overallinterface resistance by controlling the gap separating the heat sourceand sink, the nested channels effectively reduce the thermal resistanceby delaying stacking until the end of bondline formation at which timelocalized regions of increased particle density are formed. Lastly, FIG.4 also illustrates how a denser stacking pattern further decreases theoverall thermal resistance, which can easily be incorporated or designedaround the known hotspot regions of the corresponding chip. In certainembodiments, it is desirable for the nested channels to have a smallenough pitch in order to create an array of stacks with optimum thermalproperties resulting from a dense packing of small and large particleswith a minimum of void spaces between the particles. An HNC(hierarchical nested cell) cell size or channel pitch of less than 2 mmis typically used.

For the microprocessor illustrated in FIG. 1, there are two hot spots inthe lower half of the chip and one region in the upper half with anelevated temperature. FIG. 5 illustrates an example of one approach toensure a higher thermal conductivity for three regions of themicroprocessor by properly varying the channel pattern across thesurface. In FIG. 5, the dashed lines represent the edge of the chip.Typical width and depth dimensions for channels 11 are 300×300 micronsin this example. Typical width and depth dimensions for channels 12 are200×200 microns in this example. Channels 13 and 14 are smaller thanchannels 12 such as about 100-150 microns. Also, smaller channels can befurther included, if desired, such as about 10 to about 50 microns andalso nested within the triangular cells. Smaller channels are effectiveat re-directing the flow when the bondline gap is squeezed to less thanapproximately one half the hydraulic diameter of the channels. Thus, asan example, a 20 micron deep and wide channel is not useful forbondlines greater than 10 micron.

The overall concept of this invention is to design the nested channelarrays around the hotspots of the microprocessor so as to maximizeinterface thermal conductivity around these areas. Because the nestedchannels are added to the package lids/caps/heat sinks and the like, itdoes not require changes to the microprocessor fabrication process orthe manufacturing and assembly line—only a modification to one surfaceof one component in the chip package.

The best performance it typically achieved employing a paste or adhesivethat is mixed with a higher loading of particles (e.g. typically atleast about 70% by volume and more typically about 70% to about 90% byvolume) and smaller particle size range (e.g. about 0.5 to about 10microns, and more typically about 0.5 to about 2 microns) than wouldconventionally be used with two flat surfaces. The conductive particlesmay also be nano particles. The exact amount of additional particleloading depends upon the particle sizes and the matrix oil into whichthey are mixed. Such will become apparent to those skilled in the artonce they are aware of the present disclosure and need not be describedin any further detail herein. A typical matrix oil viscosity is about 5to about 20 Stokes; however, when a higher viscosity matrix oil is used(about 50 to about 200 Stokes), a higher volumetric loading of particlescan be mixed with the oil to increase performance with the nestedchannel surfaces. The thermally conductive pastes, greases or adhesivescontain particles of a highly thermoconductive material such as alumina,beryllium, copper, silver, or graphite.

The foregoing description illustrates and describes the disclosure.Additionally, the disclosure shows and describes only the preferredembodiments but, as mentioned above, it is to be understood that it iscapable to use in various other combinations, modifications, andenvironments and is capable of changes or modifications within the scopeof the invention concepts as expressed herein, commensurate with theabove teachings and/or the skill or knowledge of the relevant art. Theembodiments described herein above are further intended to explain bestmodes known by applicant and to enable others skilled in the art toutilize the disclosure in such, or other, embodiments and with thevarious modifications required by the particular applications or usesthereof. Accordingly, the description is not intended to limit theinvention to the form disclosed herein. Also, it is intended to theappended claims be construed to include alternative embodiments.

All publications and patent applications cited in this specification areherein incorporated by reference, and for any and all purposes, as ifeach individual publication or patent application were specifically andindividually indicated to be incorporated by reference.

1. A microelectronic structure comprising integrated circuit chip; aheat sink or cooling device adjacent the integrated circuit chip;wherein the heat sink or cooling device contains nested channel arrayslocated opposite the vicinity of hot spots on the integrated circuitchip; and a thermally conductive composition containing thermallyconductive particles located between the heat sink or cooling device andthe integrated circuit chip and containing a higher concentration ofconductive particles in the vicinity of hot spots on the integratedcircuit chip.
 2. The microelectronic structure of claim 1 wherein thechannels have a depth of about 10 to about 300 micrometers and a widthof about 10 to about 300 micrometers.
 3. The microelectronic structureof claim 1 wherein the hot spots are at least 2° C. higher than otherportions of the chip.
 4. The microelectronic structure of claim 1wherein the conductive particles are micro or nano particles.
 5. Themicroelectronic structure of claim 1 wherein the conductive particleshave particle size of about 0.5 micron to about 10 microns.
 6. Themicroelectronic structure of claim 1 wherein the conductive particleshave particle size of about 0.5 micron to about 2 microns.
 7. Themicroelectronic structure of claim 6 wherein the conductive compositioncontains at least about 70% by volume of the conductive particles. 8.The microelectronic structure of claim 1 wherein the conductivecomposition contains at least about 70% by volume of the conductiveparticles.
 9. The microelectronic structure of claim 1 wherein theconductive composition has a matrix viscosity of about 5 to about 20Stokes.
 10. A method for fabricating a microelectronic structurecomprising providing an integrated circuit chip; providing a heat sinkor a cooling device adjacent the integrated circuit chip; wherein theheat sink or cooling device contains nested channel arrays locatedopposite the vicinity of hot spots on the integrated circuit chip;providing a fluid thermally conductive composition containing thermallyconductive particles between the integrated circuit chip and heat sinkor cooling device; and pressing the integrated chip and heat sink orcooling device together causing the thermally conductive material toflow outward from the vicinity of the nested channel array center pointsand to thereby provide higher concentration of the thermally conductiveparticles directly over hot spot regions in the integrated circuit chip.11. The method of claim 10 wherein the channels have a depth of about 10to about 300 micrometers and a width of about 10 to about 300micrometers.
 12. The method of claim 10 wherein the hot spots are atleast 2° C. higher than other portions of the chip.
 13. The method ofclaim 10 wherein the conductive particles are micro or nano particles.14. The method of claim 10 wherein the conductive particles haveparticle size of about 0.5 micron to about 10 microns.
 15. The method ofclaim 10 wherein the conductive particles have particle size of about0.5 micron to about 2 microns.
 16. The method of claim 15 wherein theconductive composition contains at least about 70% by volume of theconductive particles.
 17. The method of claim 10 wherein the conductivecomposition contains at least about 70% by volume of the conductiveparticles.
 18. The method of claim 10 wherein the conductive compositionhas a viscosity of about 5 to about 20 Stokes.